This application claims the benefit of Korean Application No. 99-41767, filed Sep. 29, 1999, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens having a high numerical aperture (NA) for high-density optical focusing, and an optical pickup using the objective lens, and more particularly, to an objective lens for high-density optical focusing, that can be manufactured using existing techniques to have a NA high enough for high-density optical focusing, and an optical pickup using the objective lens for high-density recording.
2. Description of the Related Art
Assuming that a single objective lens is used in an optical system, the objective lens for use in recording data on and reproducing data from an optical disk has a maximum NA of 0.6 due to limitations caused by the manufacturing process. As a result, it is impossible to reduce an allowable error below an aberration of 0.07xcexrms. Examples of a conventional objective lens and an optical pickup using the conventional objective lens are shown in FIGS. 1 and 2.
Referring to FIG. 1, the conventional optical pickup for recording and/or reproducing information is designed to enable a high-density recording of 20 gigabytes on an optical disk 1. The optical pickup includes a light source 11 having a wavelength of 400 nm, a grating 19 for diffracting and transmitting the incident light beam, a first polarization beam splitter (PBS) 21 for altering the light path according to a polarization direction, a xcex/4 plate 23 for guiding a circular polarized light beam to the optical disk 1, an objective lens unit 50 having an NA of 0.85, a second PBS 27 for transmitting or reflecting the incident light beam reflected from the optical disk 1 and the first PBS 21, a first photodetector 31 for receiving the light beam that has passed through the second PBS 27 and detecting an information signal from the incident light beam, and a second photodetector 37 for receiving the light beam reflected from the second PBS 27 and detecting an error signal therefrom.
A collimating lens 13, a beam shaping prism 15, and a xcex/2 plate 17 are arranged on an optical path between the light source 11 and the grating 19. The collimating lens 13 collimates the incident light beam, the beam shaping prism 15 shapes the incident light beam, and the xcex/2 plate 17 delays the phase of the incident light beam. Another xcex/2 plate 25, which delays the phase of the incident light beam, is arranged on the optical path between the first PBS 21 and the second PBS 27. A first condensing lens 29, which condenses the incident parallel light beam, is arranged between the second PBS 27 and the first photodetector 31. A second condensing lens 33, which condenses the incident parallel light beam, and an astigmatism lens 35, which causes astigmatism, are disposed between the second PBS 27 and the second photodetector 37. A third condensing lens 39 condenses the light beam emitted from the light source 11 and reflected from the first PBS 21, and a monitoring photodetector 41 monitors the optical power of the light source 11 from the light beam condensed by the third condensing lens 39.
The objective lens unit 50 includes an objective lens 51 to focus the incident light beam and a semi-spherical lens 55, which is arranged between the objective lens 51 and the optical disk 1, to increase the NA of the objective lens unit 50.
The objective lens unit 50 uses the semi-spherical lens 55 to further increase the NA of 25 the objective lens unit 50 beyond the NA of 0.6 of the objective lens 51. Referring to FIG. 2, the NA of the semi-spherical lens 55 is proportional to the product of sin xcex8, wherein xcex8 is the maximum incident angle of light onto the semi-spherical lens 55, and the refractive index n of the semi-spherical lens 55. Thus, the NA of the objective lens unit 50 can be increased up to 0.85 using a semi-spherical 1 lens 55 and an objective lens 51.
In order to reduce the size of a light spot focused on the optical disk 1 with such a high NA in the conventional optical pickup as shown in FIG. 1, the working distance d1 between the optical disk 1 and the semi-spherical lens 55 must be as small as 0.1 mm. However, such a small distance d1 hinders the optical disk 1, preventing it from stably seating on a turntable (not shown) and from rotating during operation. In addition, if the objective lens moves in the focusing direction within the range of xc2x10.7 mm, which is beyond the working distance d1, the focusing servo control must be precisely controlled within the range of 10 nm. Thus, it is difficult to manufacture the optical pickup on a mass production scale.
To solve the above problems, it is an objective of the present invention to provide an objective lens for high-density focusing that has a high numerical aperture (NA) and ensures a sufficient working distance with respect to a recording medium, and an optical pickup using the objective lens.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
According to an aspect of the present invention, there is provided an objective lens comprising a first transmitting portion placed at a relatively near-axis region from the optical axis, that divergently transmits an incident light beam, a first reflecting portion that divergently reflects the incident light beam and faces the first transmitting portion, a second reflecting portion formed at a relatively far-axis region around the first transmitting portion that focuses and reflects the light beam reflected from the first reflecting portion, and a second transmitting portion formed at a relatively far-axis region around the first reflecting portion that refracts and transmits the light focused by the second reflecting portion as a peripheral light beam, wherein a maximum angle xcex1 between the optical axis and the peripheral light beam satisfies the condition of 30xc2x0 xe2x89xa6xcex1xe2x89xa665xc2x0.
According to another aspect of the present invention, there is provided an optical pickup comprising a light source for emitting a light beam, an optical path changing means for changing the traveling path of an incident light beam, an objective lens unit for focusing the incident light beam to form a light spot on the optical disk, and a photodetector for receiving the incident light beam reflected from the optical disk and passed through the objective lens and the optical path changing means. The objective lens unit comprises a first objective lens for focusing the incident light beam from the optical path changing means and a second objective lens arranged on the optical path between the first objective lens and the optical disk, to further focus the light beam condensed by the first objective lens, the second objective lens comprising a first transmitting portion formed at a region relatively near to the optical axis to divergently transmit the incident light beam, a first reflecting portion arranged facing the first transmitting portion, to divergently reflect the incident light beam, a second reflecting portion formed at a relatively far-axis region around the first transmitting portion, to focus the light beam reflected from the first reflecting portion toward the optical disk, and a second transmitting portion formed at a relatively far-axis region around the first reflecting portion, that diffracts and transmits the light beam condensed by the second reflecting portion.